Network ring topologies are gaining in popularity, particularly in Internet Protocol (IP) networks. Such networks enable carriers to offer large bandwidth to users in a cost-effective manner. They also lend themselves to fast rerouting in the event of network failures, since two alternative routes—in clockwise and counterclockwise directions—are generally available for connecting any two nodes on the ring. A drawback of traditional ring implementations, such as SONET/SDH, is that one of the directions is designated as the active ring, while the other direction remains on standby for fault protection when needed. In other words, at any given time, all of the nodes in the ring transmit and receive data only in the active direction. Therefore, ordinarily half of the available bandwidth in these rings is reserved for fault protection and is not exploited under normal operating conditions.
Some recently-developed bidirectional protocols provide more efficient bandwidth utilization by enabling data to be transferred between any pair of nodes in either direction around the ring, while maintaining fast protection against faults. The two opposing traffic directions are commonly referred to as an inner ring and an outer ring. It will be understood, however, that in the context of the present patent application and in the claims, the terms “inner” and “outer,” as well as “clockwise” and “counterclockwise,” are used arbitrarily to distinguish between the two opposing directions of packet flow in a ring network. These terms are chosen solely for convenience of explanation, and do not necessarily bear any relation to the physical characteristics of the network.
The leading bidirectional protocol for high-speed packet rings is the Resilient Packet Rings (RPR) protocol, which is in the process of being defined as IEEE standard 802.17. Network-layer routing over RPR is described, for example, by Jogalekar et al., in “IP over Resilient Packet Rings” (Internet Draft draft-jogalekar-iporpr-00), and by Herrera et al., in “A Framework for IP over Packet Transport Rings” (Internet Draft draft-ietf-ipoptr-framework-00). A proposed solution for Media Access Control (MAC—protocol layer 2) in bidirectional ring networks is the Spatial Reuse Protocol (SRP), which is described by Tsiang et al., in Request for Comments (RFC) 2892 of the Internet Engineering Task Force (IETF). These documents are incorporated herein by reference. They are available at www.ietf.org. Using protocols such as these, each node in a ring network can communicate directly with all other nodes through either the inner or the outer ring, using the appropriate Media Access Control (MAC) addresses of the nodes.
Neighboring ring networks are commonly interconnected through one or more interconnect nodes. Similarly, an interconnect node on a ring network may connect the ring to another network having a different topology. When the interconnect node links a spur or access ring to a main trunk network, the interconnect node is also referred to as a gateway. In cases in which the interconnect node links two ring networks, the node must typically support four times the bandwidth of a single (clockwise or counterclockwise) ring.
When an interconnect node on a ring network fails, the ring network can become isolated from the other networks to which it is normally connected. Even if multiple interconnect nodes are used to connect two networks, a failure in one of the nodes will affect the connections between the networks that previously passed through the failed node. This problem can be solved by installing an additional node as backup to each interconnecting node, and activating the backup node when the node that is normally active fails. In other words, two nodes are installed at the same site, at substantial added cost and demanding additional space, power and maintenance. Duplication of the node also requires that the active and standby nodes follow a mirroring protocol, so that the standby node will be ready to take over when needed. Because each of the two nodes must be able to carry the full bandwidth of the network interconnection that it supports, the backup node cannot practically be used for load sharing. Therefore, the resources of the backup node are wasted most of the time.